Field of the Invention
The invention relates to a liquid crystal alignment agent, a liquid crystal alignment film, and a liquid crystal display element. More particularly, the invention relates to a liquid crystal alignment agent capable of forming a liquid crystal alignment element having good resistance to ultraviolet decay and no mura defect, a liquid crystal alignment film formed by the liquid crystal alignment agent, and a liquid crystal display element having the liquid crystal alignment film.
Description of Related Art
With the development of the liquid crystal display toward a large-size display specification, in order to overcome the issue of viewing angle of a large-size display, the wide-viewing angle techniques of the liquid crystal display panel also need to continuously progress and advance. The multi-domain vertical alignment (MVA)-type liquid crystal display panel is a common wide-viewing angle technique. In the MVA-type liquid crystal display panel, a protrusion is formed in the liquid crystal panel, and the protrusion can limit the tilting direction of a liquid crystal molecule, thereby achieving the display effect of wide-viewing angle. However, the MVA-type liquid crystal display panel cannot prevent the issues of insufficient transmittance and contrast and slow response speed of the liquid crystal molecule originating from the protrusion.
In recent years, to solve the above issues, a polymer-sustained alignment (PSA)-type technique has currently been developed. In the technique, a liquid crystal composition containing a polymerizable compound is clamped in the gap of a pair of substrates formed by a substrate having a patterned conductive film and a substrate without a patterned conductive film or in the gap of a pair of substrates formed by two substrates each having a patterned conductive film, and ultraviolet is irradiated on the liquid crystal composition when voltage is applied between the conductive films to perform polymerization on the polymerizable compound. As a result, pre-tilt angle characteristics are exhibited, and the direction of liquid crystal alignment is controlled. The technique can make the conductive films form a specific structure to achieve the effects of wide-viewing angle and increased response speed of a liquid crystal molecule, thus solving the inevitable issues of insufficient transmittance and contrast of the MVA-type liquid crystal display panel.
However, to generate polymerization to the polymerizable compound, a large amount of ultraviolet of, for instance, 100,000 J/m2 needs to be irradiated. Therefore, in addition to the concern that liquid crystal molecules may be decomposed, unreacted compounds that are not polymerized from ultraviolet irradiation also remain in the liquid crystal layer to form impurity contamination, such that the issue of mura defect is generated to the liquid crystal display element, which in particular has a negative effect on electrical characteristics. Moreover, the type of the liquid crystal molecule used in the liquid crystal layer also needs to correspond to the added polymerizable compound, and is therefore limited.
Therefore, how to solve the issue of impurity contamination of the liquid crystal layer and provide a liquid crystal display element having good resistance to ultraviolet decay and no mura defect is a current topic actively researched by those skilled in the art.